1. Field of the Invention
The present invention relates to a waveform generator which is used in CRT display unit or the like and adapted to generate saw-tooth waves and parabolic waves.
2. Description of the Prior Art
FIG. 1 is a schematic diagram showing a conventional waveform generator. In the figure, indicated by 13 is a comparator, 2 is a resistor connected to a D.C. power source 1, 3 is a capacitor connected to the resistor 2, 4 is a transistor having its emitter grounded, its collector connected to the node between the resistor 2 and capacitor 3, and its base connected to the output of the comparator 13. The comparator 13 has its one input terminal connected to the node between the resistor and capacitor mentioned above and another input terminal connected to a reference voltage source 14. Indicated by 15 is an integrator formed of a double-balanced differential amplifier or the like, with its input terminal connected to the above-mentioned node which is the output terminal c of saw-tooth waves. The integrator 15 has an output terminal indicated by d.
FIG. 2 is a timing chart showing saw-tooth waves produced on the output terminal c shown in FIG. 1 and a parabolic wave produced on the output terminal d of the integrator 15. Shown by c5 is a saw-tooth waveform having a period of T, c6 is a saw-tooth waveform having a period of 2/3T, and d2 is a parabolic waveform.
Next, the operation will be explained. With the transistor 4 being in a cutoff state, the capacitor 3 is charged by the D.C. power source 1 through the resistor 2. Consequently, the voltage of the output terminal c rises progressively along the charging curve which is the saw-tooth wave c5 shown in FIG. 2. When the voltage becomes equal to the reference voltage 14 which is applied to one input terminal of the comparator 13, it produces an output signal on its output terminal. Receiving the comparator output on the base, the transistor 4 becomes conductive, and it discharges the capacitor 3 through the collector and emitter, causing the voltage of the output terminal c to fall to zero. In order to get the saw-tooth wave output c6 having a period shorter than that of the saw-tooth wave c5, the magnitude of the current flowing through the resistor 2 is varied.
In this manner, the oscillation frequency of saw-tooth wave can be set arbitrarily by varying the current of the resistor 2, with the values of the resistor 2 and capacitor 3 being set constant. Through the integration of the saw-tooth wave output c5 or c6 with the integrator 15, a symmetrical parabolic waveform as shown by d in FIG. 2 is produced.
However, the conventional waveform generator arranged as described above needs to vary the current of the resistor 2 for changing the oscillation frequency, which cannot be done easily. Moreover, CRT display units have the additional provision of an S-shaped modification circuit for the correction of linearity in the vertical direction and a PCC circuit for the correction of pincushion distortion, and these circuits used with the conventional waveform generator are expensive and are not responsive to a changing frequency.